1. Field of the Invention
This invention relates to the processing of sinterable colloidal materials, such as ceramics and metals, in suspension. More particularly, this invention relates to providing highly volume loaded (.gtoreq.55 v/o) suspensions of such materials which nevertheless are readily flowable, such as by pouring or by low pressure injecting.
2. The state of the art
Advances in ceramic processing have permitted the replacement of various components of electrical and mechanical equipment with sintered ceramic parts. Ceramics have found widespread use in electronics as a substrate for integrated circuit packages. Ceramic substrates are typically formed by sintering slips produced from suspensions of ceramic particles. however, ceramics have exhibited problems of cracking, large and inconsistent shrinkage upon drying and firing, and non-uniform formability leading to poor microstructures.
It is well-known that processing defects can greatly affect the performance of a ceramic article. See, e.g., Bowen et al., "Basic Needs in High Temperature Ceramics," J. Mat. Sci. Eng., vol. 44, p. 156ff (1980); Rhodes, "Agglomeration of Particle Size Affects Sintering of Y.sub.2 O.sub.3 -Stabilized ZrO.sub.2," J. Amer. Cer. Soc., vol. 64, pp. 19-22 (1981); Lange et al., "Processing Related Fracture Origins: I, II, III," J. Amer. Cer. Soc., vol. 65, pp. 396-408 (1983); and Aksai et al., "Uniformity of Alumina-Zirconia Composites by Colloidal Filtration," J. Amer. Cer. Soc., vol. 66, no. 10, pp. C190 and C193; and see generally Ceramics Processing Before Firing, Ed. by Onoda, Jr., and Hench (New York: John Wiley & Sons, 1978). These processing defects are typically property limiting; however, the above investigators have demonstrated that by wet processing of colloidal ceramic particles, processing flaws can be virtually eliminated. This is typically achieved by controlling the packing of the particles on a particulate scale and not on a macro part-sized scale.
Rheology characteristics are a function of the volume fraction of solids; for example, viscosity, or rate of momentum transfer, or flowability, or pourability, is directly related to the "free volume" between particles. (Accordingly, to represent a volume fraction the symbol "v/o" will be used herein as a contraction of "vol. %".) Where V.sub.fs is the volume fraction of solids, 1-V.sub.fs is the free volume; as Albert Einstein showed in 1925 for dilute solutions of spheres in liquid, N.sub.r =1+2.5 V.sub.fs, where N.sub.r is the viscosity of the slurry relative to the viscosity of the liquid without suspended particles (i.e., the relative viscosity). The relative viscosity can also be referenced to the maximum packing concentration of solids, V.sub.max, the closest packed arrangement of particles in a dense, randomly packed array, as shown by Chong et al., J. Appl. Polymer Sci., vol. 15, p. 2007 (1971). In this reference frame, N.sub.r =(1+0.75(V.sub.fs /V.sub.max)/(1-(V.sub.fs /V.sub.max))).sup. 2 ; accordingly, as the volume fraction of solids approaches the maximum, the viscosity becomes infinite.
With regard to the actual particles in suspension, a distinction exists between colloids and non-colloidal particles; the former being governed by surface forces and therefore acting as individual flow units, while the latter are governed by gravity or body forces (e.g., drag). There are a number of factors effecting whether a particle is colloidal, including actual size, density, and geometry. A rough estimate is that particles having an equivalent spherical diameter of .ltoreq.5 .mu.m are colloidal; however, colloidal particles can be 20 .mu.m or 50 .mu.m in size, and metal particles exhibit colloidal properties at larger sizes than do oxide ceramics because they also exhibit a higher Hamaker constant.
Nevertheless, in all cases, particle surfaces (colloidal or otherwise) are characterized by broken crystallographic bonds which were cleaved during particle synthesis. These broken bonds have a high free energy and provide reactive sites due to the desire of these surface atoms to be bonded to another species. This surface reaction zone, which extends into the dispersion medium at the interface of the particle-fluid boundary, is called the diffuse double layer (DDL). The DDL is considered to be attached to the particle surface, so that as the particle moves through a fluid, it carries the DDL with it. For colloidal particles, the volume of the double layer can be on the order of the particle diameter, such that a significant amount of volume for the flow unit can be attributed to the DDL. A quantity, V.sub.eff, the effective volume solids, can be described as V.sub.eff =V.sub.fs +V.sub.DDL, where the latter term is the volume of the adsorbed double layer. The unsatisfied surface charges of the particle are felt throughout the DDL in a distribution which is strongest at the particle surface and is typically modeled to exponentially decrease out into the bulk solution.
For colloidal dispersions, the interparticle double layer interactions affect the viscosity of the dispersion at a lower volume fraction of solids than for non-colloidal dispersions because the double layer volume increases the effective volume fraction solids and its corresponding effect on the pourability of the dispersion.
Interparticle forces can be either attractive, such as Van der Waals attractionn (which is largely a function of material type and particle-to-particle distance), or repulsive, as when particles having the same surface charge are brought closer together. The double layer volume can be reduced by neutralizing or satisfying the surface charge on the particles. For example, dispersants (surface active soluble species which adsorb onto particle surfaces) are attracted to the double layer, and by absoring to the charged surface molecules act to minimize the free energy of the interfacial region; however by doing so they decrease the double layer, which may allow attractive forces to dominate. Thus, where repulsive forces used to dominate and create a high viscosity due to a large effective volume fraction of solids, attractive forces may dominate and particle agglomeration may occur with an accompanying increase in viscosity due to attractive agglomerative interactions, and the agglomerates may attract each other further exacerbating the situation. For a practical point of view, one may have a high viscosity system, add an amount of surfactant, and still have a high viscosity system, even though (unnoticed) the system chemistry is completely different; adding an amount of surfactant effective to achieve the desired viscosity is empirical, so a lesser amount of surfactant added may have lowered the viscosity. In general, to create a high solids concentration colloidal suspension which is nevertheless fluid and pourable, it is essential to provide a suspension in which the particles are predominantly non-interacting and thus act, essentially, as individual flow units.
Dispersants play an important role in keeping the double layer small while overcoming the Van der Waals attractive forces, and the use of polymeric polyelectrolytic dispersants to achieve highly loaded colloidal suspensions for ceramics or powdered metal manufacture is not generally known in the literature. Highly loaded, non-colloidal dispersions are known in the art. For example, Klimenco and Polyakov, Glass and Ceramics, vol. 43.4, pp. 165-167 (1986), describe a technique for making highly concentrated silicon carbide suspensions in water at a 50% solids concentration; however, these particles were predominantly non-colloidal. Non-colloidal particles are large enough so that the V.sub.DDL has a negligible effect on the effective particle volume, V.sub.eff. For example, a 20 .mu.m diameter alumina sphere will have a particle volume of 1.33.times.10.sup.-9 cm.sup.3. Napper, Polymeric Stabilization of Colloidal Dispersions, p. 424 (New York; Academic Press, 1983) has shown that a typical low molecular weight polymeric polyelectrolyte (e.g., 10,000 MW) will give an adsorbed layer of approximately 5 nm. Thus, when such a dispersant is adsorbed onto a 20 .mu.m particle, the polyelectrolyte double layer will have a negligible affect on the effective particle volume. On the other hand, colloidal particles are small enough so that the double or adsorbed layer surrounding the particle is a significant portion of the effective particle volume. For example, a spherical 0.2 .mu.m diameter particle will occupy a volume of 4.19.times.10.sup.-15 cm.sup.3. Assuming the same adsorbed layer of 5 nm, the effective radius of the particle with an adsorbed layer is 0.105 .mu.m, thus resulting in an effective spherical volume of 4.85.times.10.sup.-15 cm.sup.3. This represents a 15.8% increase in the effective volume of each particle, which thus translates to an equivalent increase in the effective volume fraction solids. That is, the "sphere of influence" of the double layer of an alumina colloidal particle can increase the effective volume by which the particle interacts with other particles by almost 16%, a tremendous effect on relative viscosity.
Commercial colloidal processing has experienced several difficulties in the formation of ceramic parts due to one or more of the following factors:
1. Due to the inability to overcome interfacial and wetting forces of fine particulates, which serve to keep particles as discrete suspension units, processing has typically been limited to dilute suspensions.
2. Typical dilute suspensions result in a low green density, leading to longer drying and binder burnout, increased shrinkage and stresses therefrom on firing, and inhomogeneous microstructures, all leading to nonreproducible processing and properties.
3. Dilute suspensions tend to have very slow rate-limiting processing steps. Dewatering or debinderizing the carrier between fine particles is generally very slow due to the permeability, settling, sedimentation, and diffusion limitations for compacts of these fine powders and pore sizes.
If these obstacles were overcome, highly loaded, low viscosity, pourable suspensions would be a valuable feedstock for overcoming these processing limitations.
Phelps and McLaren, "Particle-Size Distribution and Slip Properties" in Ceramics Processing Before Firing, op. cit., note certain rheology characteristics of both uniformly sized particles and colloidal particles. At a loading of 51 v/o for a coarse alumina slip, a shear thickening result is evident, and increasing the loading of a fine alumina powder from 50 v/o to 57 v/o results in a dramatic decrease in flow for a given force. The effect of colloidal particles has been discussed above, and Phelps and McLaren conclude that the more extended a distribution of particles (i.e., the less uniformly sized), the more fluid the slip is at high volume solids loadings. As they show, even "non-colloidal" but uniformly sized particles are not flowable at loadings greater than about 51 v/o. Generally, the fine particulate ceramics industry considers 40-45 v/o solids to be "concentrated" or highly loaded. See, e.g., Sommer, "Viscosity of Concentrated Newtonian Suspensions," Ibid. at pp. 227-233.
With such a background, even the present state of the art must empirically determine suitable dispersants by an empirical method, typically by first deciding on a solvent system (e.g., aqueous, non-aqueous) and then empirically choosing a series of conventional dispersing agents to test in the suspension system. For example, Mikeska and Cannon, "Dispersants for Tape Casting Pure Barium Titanate," Advances in Ceramics: Forming of Ceramics, Volume 9, ACS Publishing, p. 164-183 (1984), screen over 70 commercially available dispersants before deciding on a satisfactory system for their particular materials. Similarly, Nilsen and Danforth, "Dispersion of Laser-synthesized Silicon Nitride Powder in Nonaqueous Systems," Advances in Ceramics: Ceramic Powder Processing, Volume 21, ACS Publishing, pp. 537-547 (1987), screened over 25 dispersants before arriving at a satisfactory system.
Dispersants are generally postulated to function by at least one of two mechanisms. Steric dispersants are understood to operate by presenting (i) functional groups exhibiting strong surface interaction with the particle surface while being only marginally soluble in the solvent and (ii) stabilizing moieties that are highly soluble in the solvent. Electrostatic dispersant include acids and bases which operate to modify the pH of the suspending medium. Typically, dispersing agents have been used in aqueous and other solutions to create pourable suspensions of submicron particles having a maximum solids fraction of approximately 50 v/o. Although this maximum solids fraction can be higher if the particles do not have a narrow size range, uniform size particles may be desirable to make for uniform ceramics having uniform properties, such as reproducible shrinkage for net or near net shape forming, and for high performance applications.